In telecommunication systems, especially in the field of telephony, alternating-current signals such as voice-frequency oscillations are sent over an unbalanced two-wire line as modulations of a direct current normally present on that line in its operating condition, the incoming signals being inductively communicated to a receiver such as an earphone through a so-called hybrid-coil transformer which decouples that receiver from an associated transmitter such as a microphone. Unless the signal-transmitting circuit is properly balanced with reference to the line, its normally unshielded output leads may capacitively pick up stray signals from the adjoining input leads of the receiving circuit which may generate parasitic oscillations on the line loop. Furthermore, the inductive coupling prevents the use of integrated circuitry and consequently requires a relatively bulky and weighty structure. Finally, unavoidable circuit losses (e.g. in a matching impedance) prevent the use of 100% modulation of an available direct current in the signal-transmitting network, which limits the permissible line length between a subscriber station and a central office or a repeater.
In my copending application Ser. No. 560,517 of even date I have disclosed a purely resistive hybrid circuit which can be readily integrated and also provides a symmetrical structure for the suppression of line noise. With such a hybrid circuit it is necessary that, in order to avoid low-resistance shunts, the impedance across the output terminals of the signal-transmitting network and the input terminals of the signal-receiving network be high. The more recently developed electroacoustic transducers with piezoceramic membranes, because of their high sensitivity, are eminently suitable as a microphone and an earphone in a signal-transmitting network and a signal-receiving network of high terminal impedance coupled to the line by such a hybrid circuit.